The Centers for Disease Control and Prevention (CDC) issued updated guidance last week for people to wear masks indoors in public areas, regardless of vaccination status, where community spread is substantial or high. The CDC’s map uses different measures to determine the rate of transmission than measures the Idaho’s COVID-19 Dashboard depicts. Idaho’s dashboard is maintained by the Division of Public Health in the Department of Health and Welfare.

We strongly encourage everyone to follow the CDC guidance and wear a mask indoors in public areas where the rate of transmission is substantial or high. The only way we’ll beat the virus that causes COVID-19 is by limiting its ability to spread and mutate yet again into another variant of public health concern. We’re in a race against the virus, and vaccination is still the best protection against serious illness, hospitalization, and even death from COVID-19. But the Delta variant is a worthy foe and vaccination rates aren’t high enough to keep it in check. So wearing a mask is important again.

But we also understand how confusing it is that the data on the CDC’s site and on Idaho’s dashboard don’t match, and we’d like to explain why that is.

The CDC is using case rates or molecular testing percent positivity to calculate the community rate of COVID-19 transmission. Here are explanations for why the data are different on both sites.

CASE RATES

• The CDC case rates are based on a 7-day total of cases.
• DHW rates are based on a 7-day daily average of cases.

Here’s the CDC method for calculating the case rate:

Example case counts:

Day	Case counts
Sunday	0
Monday	100
Tuesday	150
Wednesday	175
Thursday	200
Friday	125
Saturday	100

On Saturday, CDC adds all the case counts together for a total of 850 cases.

They divide that total by the population, and then multiply it by 100,000 for a 7-day (weekly) rate:

If the population is 55,000 people, the calculation would look like this:

(850 cases / 55,000 people)  = 0.015455

(100,000 x 0.015455)  = 1,545.46 per 100,000 people (7-day rate)

Here’s the DHW method for calculating case rates, using the same sample case numbers above:

On Saturday, DHW adds all the case counts together for a total of 850 cases. We divide by 7 to determine the daily average. We divide the daily average by the population and then multiply by 100,000 for a 7-day average daily rate.

(850 cases / 7 days) = 121.43 cases per day

121.43 average cases / 55,000 people = 0.002208

(100,000 x 0.002208) = 220.78 per 100,000 people (7-day average daily rate)

TESTING PERCENT POSITIVITY

DHW bases percent positivity on the date the specimen was collected. Every week, DHW staff verify every single laboratory result record to determine the patient’s residence and exclude non-Idaho residents from the data. DHW also removes any duplicate records and excludes positive results from the numerator if the reporting laboratory does not also provide negative results because including those positive results would artificially inflate our percent positivity.

• On average:
  • 1% of the records are duplicate records
  • 10% of the records received are results for non-Idaho residents
  • 15% of the records do not include the patient’s county of residence
  • 10% of the records have no address information
  • <1% of the positive results come from laboratories that don’t provide negative results.

CDC uses the date the lab report was received by the public health agency, so the timeframe is different than DHW’s data. They use less precise methods to determine the patient’s county of residence when it isn’t available in the record. CDC also includes results from laboratories that send positive results, but do not send negative results. They also do not receive identifiable information and are unable to:

• Determine duplicate records
  • Exclude lab results for non-Idaho residents without address information

The result of these differences is that CDC percent positivity data are comparable between different states, but are not comparable to the percent positivity data generated by DHW.

• Dates and timeframes are not the same
• A proportion of the CDC data are based on less precise methods to determine residency
• CDC does not verify each record, which reduces the precision and accuracy of their calculations

Although the CDC case count data and percent positivity are not necessarily comparable to DHW COVID-19 data in real time, it is critical that the CDC have some way to compare trends across states and counties in the US. Each state has their own dashboard and may display different measures. For the purpose of maintaining nationwide awareness of the COVID-19 pandemic, the data CDC are using is adequate and we support the use of the CDC data for that purpose.   

Dr. Kathryn Turner is the deputy state epidemiologist in the Division of Public Health. She and her team collect, analyze, and publish COVID-19 data during the pandemic.

The Idaho Department of Health and Welfare is dedicated to strengthening the health, safety, and independence of Idahoans. Learn more at healthandwelfare.idaho.gov.